Power converters are key components in many military and commercial systems and they often govern size and performance. Power density, efficiency and reliability are key characteristics used to evaluate the characteristics of power converters. Transformers and inductors used within these power converters may be large and bulky and often limit their efficiency, power density and reliability. These deficiencies can be improved by using a high frequency “switch-mode” architecture instead of a traditional step-down configuration and by replacing conventional core-and-coil designs with “planar magnetics.”
Planar magnetics offer several advantages, especially for low-power do-dc converter applications, such as low converter profile, improved power density and reliability, reduced cost due to the elimination of discrete magnetic components, and close coupling between different windings. For example, the integrated magnetics 10 shown in FIG. 1 for a current-doubler rectifier (CDR) comprises an E-core 12 and plate 14 wound with split-primary windings 16 and 18, secondary windings 20 and 22, and an inductor winding 24 (See U.S. Pat. No. 6,549,436). This type of core arrangement is referred to as an E-I core. Other core geometries, for example circular core legs, are also possible. The windings perform the functions of both the transformer and the two inductors used in the CDR. The center-leg winding is used to increase the effective filtering inductance and carries the full load current all the time. Gapping of the center leg is typically done to prevent core saturation.
As shown in FIGS. 2a and 2b, integrated magnetics 10 is implemented with a multi-layer printed circuit board (PCB) 26 having copper traces that form the various “horizontal” windings in the plane of the PCB. Horizontal windings refer to the configuration in which 30 the winding is oriented parallel to the core plate. In one embodiment, E-core 12 is positioned underneath the PCB so that its outer legs 28 and 30 extend through holes in the PCB that coincide with the centers of primary and secondary windings 16 and 20 and 18 and 22, respectively, and its center leg 31 extends through a hole that coincides with inductor winding 24. Plate 14 rests on the outer legs forming air gap 32 with the center leg. In another embodiment, the E-I core is attached to the circuit board and the winding terminations are attached to the circuit board traces to complete the circuit.
The coupling between the windings in the planar PCB-based magnetic structure is very strong due to their large overlapping surface areas. Tight coupling between the transformer primary and secondary windings is desirable because it minimizes leakage inductance. However, the large interwinding capacitance between the outer-leg windings (16, 20, 18 and 22) and the center-leg inductor winding 24 may provide a low-impedance path from the integrated magnetic windings directly to the output of the converter, making the inductor windings ineffective in attenuating high-frequency noises and ripple.
Another potential disadvantage of a conventional planar winding design for integrated magnetics is the large number of layers in the PCB needed to accommodate all the windings. The integrated magnetics 10 shown in FIGS. 1 and 2 uses eight PCB layers: 2 for the primary with windings connected in series, 3 for the secondary with windings connected in parallel, and two inductor windings around the center-leg. One layer is used as a ground plane. More layers are desirable from an efficiency standpoint but will be more expensive and the capacitance between the center-leg winding and the outer leg windings will increase dramatically. Furthermore, as more layers are used and the thickness of the PCB increases, some of the winding layers will inevitably be close to air gap 32 where they will suffer from high eddy current losses due to the strong fringing flux surrounding the air gap. As shown in FIG. 3, the flux lines 34 are generally perpendicular to the plane of the horizontal windings in most of the window area of the magnetic core and thus induce large eddy currents 36 in the windings. Finally, since the center-leg winding carries the full load current, its resistance and mean length per turn is an important factor in determining the overall efficiency of the converter, especially in high current applications.